1. Statement of the Technical Field
The inventive arrangements relate generally slot antennas.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the dielectric constant.
For the purposes RF circuits, it is generally important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, signal reflections and inefficient power transfer can result. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting circuit performance relate to the dielectric constant (sometimes referred to as the relative permittivity or ∈r) and the loss tangent (sometimes referred to as the dissipation factor or δ) of the dielectric substrate material. The dielectric constant determines the electrical wavelength in the substrate material, and therefore the electrical length of transmission lines and other components disposed on the substrate. The loss tangent determines the amount of signal loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes.
In general, the characteristic impedance of a parallel plate transmission line, such as stripline or microstrip line, is approximately equal to √{square root over (Ll/Cl)}, where Ll is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the dielectric constant of the dielectric material(s) used to separate the transmission lines.
In conventional RF designs, a substrate material is selected that has a single dielectric constant and relative permeability value, the relative permeability value being about 1. Once the substrate material is selected, the line characteristic impedance value is generally exclusively set by controlling the geometry of the line, the slot, and coupling characteristics of the line and the slot.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate, such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors, such as copper, gold, or tantalum, which generally function as transmission lines (e.g. stripline or microstrip line or twin-line) in the frequency ranges of interest.
The dielectric constant of the selected substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is reasonably suitable for all of the various passive components, radiating elements and transmission line circuits to be formed on the board.
In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to have an electrical length of a quarter of a wavelength. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation for a given substrate. Since the physical size of the microstrip line or stripline is inversely related to the dielectric constant of the dielectric material, the dimensions of a transmission line or a radiator element can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a high dielectric constant with values such as 50 to 100. However, the use of a dielectric with a high dielectric constant will generally result in a significant reduction in the radiation efficiency of the antenna.
Antenna elements are sometimes configured as microstrip slot antennas. Microstrip slot antennas are useful antennas since they generally require less space, are simpler and are generally less expensive to manufacture as compared to other antenna types. In addition, importantly, microstrip slot antennas are highly compatible with printed-circuit technology.
One factor in constructing a high efficiency microstrip slot antenna is minimizing the power loss, which may be caused by several factors including dielectric loss. Dielectric loss is generally due to the imperfect behavior of bound charges, and exists whenever a dielectric material is placed in a time varying electromagnetic field. The dielectric loss, often referred as loss tangent, is directly proportional to the conductivity of the dielectric medium. Dielectric loss generally increases with operating frequency.
The extent of dielectric loss for a particular microstrip slot antenna is primarily determined by the dielectric constant of the dielectric space between the radiator antenna element (e.g., slot) and the feed line. Free space, or air for most purposes, has a relative dielectric constant and relative permeability approximately equal to one.
A dielectric material having a relative dielectric constant close to one is considered a “good” dielectric material as a good dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant substantially equal to the surrounding materials is used, the dielectric loss due to impedance mismatches is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip slot antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator antenna slot and the microstrip feed line exciting the slot.
Furthermore, the use of a material with a lower dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the radiation efficiency of the microstrip slot antenna. However, the use of a dielectric material having a low dielectric constant can present certain disadvantages, such as the large size of the slot antenna fabricated on a low dielectric constant substrate as compared to a slot antenna fabricated on a high dielectric constant substrate.
The efficiency of microstrip slot antennas is compromised through the selection of a particular dielectric material for the feed which has a single uniform dielectric constant. A low dielectric constant is helpful in allowing wider feed lines, that result in a lower resistive loss, to the minimization of the dielectric induced line loss, and the minimization of the slot radiation efficiency. However, available dielectric materials when placed in the junction region between the slot and the feed result in reduced antenna radiation efficiency due to the poor coupling characteristics through the slot.
A tuning stub is commonly used to tune out the excess reactance in microstrip slot antennas. However, the impedance bandwidth of the stub is generally less than both the impedance bandwidth of the radiator and the impedance bandwidth of the slot. Therefore, although conventional stubs can generally be used to tune out excess reactance of the antenna circuit, the low impedance bandwidth of the stub generally limits the performance of the overall antenna circuit.